The dinosaurs might have gone out with a sudden bang, but their rise to dominance was a gradual ascent, not a sudden takeover, a study suggests.

It shows that dinosaurs co-existed with a more primitive group of reptiles for millions of years before becoming the most common land animals on Earth.

Experts had thought that once dinosaurs emerged, they swiftly replaced their relatives the dinosauromorphs.

But the latest study in Science journal questions this idea.

Dinosaurs first appeared around 230-220 million years ago, towards the end of the Triassic Period.

By the beginning of the Jurassic Period, about 200 million years ago, they had become the dominant creatures on land - and would remain so for another 135 million years.

We certainly know now that dinosaurs didn't take over immediately after they evolved

Dr Richard Butler, Natural History Museum

But the reasons for their success - and why the dinosauromorphs faded away - are poorly understood.

This is because the fossil record leading up to the Jurassic is relatively sparse.

Some researchers had assumed that dinosaurs were quick to supplant their more primitive relatives, the dinosauromorphs.

In other words, they got a "lucky break".

Pace of change

Dinosaurs may have either outcompeted their reptilian relatives for resources, or taken advantage of some catastrophe that devastated the Dinosauromorpha.

But fossils described in the latest issue of Science suggest that if competition occurred, it was over a prolonged period, according to the team that unearthed them in the south-western US.

The primitive reptile Dromomeron romeri is new to science

The Late Triassic cache from Hayden Quarry, New Mexico, contains primitive dinosaurs and several species of dinosauromorph - including the newly identified species Dromomeron romeri.

The researchers also found a diverse range of other animals, including amphibians, fish and crocodile-like reptiles.

These, along with other contemporary fossil "assemblages", imply the rise of the dinosaurs throughout the Triassic was a more gradual process.

Randall Irmis, from the University of California, Berkeley, US, and colleagues suggest the dinosaurs may have co-existed with the dinosauromorphs for some 15-20 million years.

"Finding dinosaur precursors - or basal dinosauromorphs - together with dinosaurs tells us something about the pace of changeover," said Mr Irmis.

"If there was any competition between the precursors and dinosaurs, then it was a very prolonged competition."

Important findings

Dr Richard Butler, a dinosaur specialist from London's Natural History Museum, UK, called the study and the discoveries "very important".

"The authors believe that their discovery demonstrates that the dinosaur takeover occurred gradually rather than rapidly. We certainly know now that dinosaurs didn't take over immediately after they evolved," he told the BBC News website.

But Dr Butler said there were still signs of rapid change at the conclusion of the Triassic.

"The big question is what happened right at the end of the Triassic period. At this point we see one or more major extinction events, with all of the primitive dinosaur-like and crocodile-like reptiles becoming extinct, and only dinosaurs, crocodiles and pterosaurs (flying reptiles) surviving."

Exactly when and how these extinction events occurred is poorly understood; but asteroid strikes and climate change have been implicated.

"These extinctions may have opened up ecological space for dinosaurs to expand into, and might have occurred in a relatively short period of time," Dr Butler explained.

"So in this sense I think it is still possible that early dinosaurs could have benefited from a 'lucky break'."